1. Field of the Invention
The present invention relates to a semi-conductor memory device removably attachable to an external circuit for performing data reading/writing operation, and particularly relates to a device such as a so-called IC card or memory card although the invention is not limited to an IC card or memory card.
2. Description of the Related Art
IC cards are generally divided into two types, one including at least computer function elements such as a CPU and a semiconductor memory so as to provide a high-degree data processing function such as arithmetic operation and the other including merely a semiconductor memory and a read/write control circuit, as shown in U.S. Pat. No. 4,888,773. The present invention relates to the IC cards of the latter type in view of the mode of practical use.
The conventional IC cards of the latter type are generally employing any of ROM, PROM, EEPROM, and S-RAM (static RAM), as a semiconductor memory. Due to difficulty in writing information readily and rapidly, the IC card employing ROM, PROM, or EEPROM is not suitable for use in the mode in which data is written data renewal) by a user after the card has been manufactured. On the other hand, the IC card using S-RAM is suitable for use in the mode in which data is written (rewritten) by a user because the data read/ write operation can be carried out therein readily and rapidly in comparison with the IC card using PROM.
The S-RAM, however, involves a problem that the number of memory bits per chip area is so small that an IC card, that is, a memory device becomes expensive. The low memory density and expensiveness of S-RAM is generally a bar to reduction in size and cost of an IC card employing such an S-RAM when the IC card is used as temporary image-data-storing means, for example, such that image data of a CCD camera is written in the IC card, and the written data is inputted into a personal computer so as to be displayed on a CRT, to be subject to graphic processing, or to be recorded on a floppy disk, or the IC card is loaded on a reader of a printer so that the image stored in the memory is printed out.
In this regard, a dynamic RAM (D-RAM) is inexpensive and has a high memory density, and the dynamic RAM is suitable for the above uses in general. In the dynamic RAM, however, it is necessary to refresh the dynamic RAM periodically at a period not longer than a predetermined unit time so as to reproduce and hold the stored information. When an IC card is connected to an external circuit, that is, an apparatus for writing/ reading data onto/from the IC card, such as an image data processing circuit of a CCD camera, an image data processing circuit of a personal computer, a reader of a printer, or the like, the data is not erased because the external circuit refreshes the dynamic RAM. In this case, however, if the IC card is separated from the external circuit, there is a problem that the data may disappear.